The invention relates to a nickel base alloy.
This invention relates to nickel-based alloys, especially for those used as a coating for high temperature gas turbine blades and vanes.
Wide use of single crystal (SX) and directionally solidified (DS) components has allowed increased turbine inlet temperature and therefore turbine efficiency. Alloys, specially designed for SX/DS casting, were developed in order to make a maximum use of material strength and temperature capability. For this purpose modem SX alloys contain Ni and solid-solution strengtheners such as Re, W, Mo, Co, Cr as well as xcex3xe2x80x2-forming elements Al, Ta, Ti. The amount of refractory elements in the matrix has continuously increased with increase in the required metal temperature. In a typical SX alloys their content is limited by precipitation of deleterious Re-, W-or Cr-rich phases.
High temperature components are typically coated to protect them from oxidation and corrosion. In order to take full advantage of increased temperature capability and mechanical properties of SX/DS blade base material, coating material must provide now not only protection from oxidation and corrosion, but must also not degrade mechanical properties of base material and have a stable bond to substrate without spallation during the service. Therefore requirements for advance coatings are:
high oxidation and corrosion resistance, superior to those of the SX/DS superalloys;
low interdiffusion of Al and Cr into the substrate to prevent precipitation of needle-like phases under the coating;
creep resistance comparable to those of conventional superalloys, which can be achieved only with the similar coherent xcex3-xcex3xe2x80x2 structure;
low ductile-brittle transition temperature, ductility at low temperature;
thermal expansion similar to substrate along the whole temperature range.
The coating described in U.S. Pat. No. 5,043,138 is a derivative of the typical SX superalloy with additions of yttrium and silicon in order to increase oxidation resistance. Such coatings have very high creep resistance, low ductile-brittle transition temperatures (DBTT), thermal expansion coefficients equal to those of the substrate and almost no interdiffusion between coating and substrate. However, the presence of such strengtheners as W and Mo, as well as a low chromium and cobalt content typical for the SX superalloys, has a deleterious effect on oxidation resistance. European Patent Publication 0 412 397 A1 describes a coating with significant additions of Re, which simultaneously improves creep and oxidation resistance at high temperature. However, the combination of Re with a high Cr content, typical for traditional coatings, results in an undesirable phase structure of the coating and interdiffusion layer. At intermediate temperatures (below 950-900xc2x0 C.), the xcex1-Cr phase is more stable in the coating than the xcex3-matrix. This results in a lower thermal expansion compared to the base material, a lower toughness and possibly a lower ductility. In addition, a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which makes it prone to precipitation of needle like Cr-, W- and Re-rich phases.
Accordingly, one object of the invention is to provide an nickel base alloy which is designed to combine an improved ductility and creep resistance, phase stability of coating and substrate during service, phase structure and thermal expansion similar to the substrate and an excellent oxidation resistance.
The invention provides a nickel base alloy, particularly useful as a coating, which comprises: (measured in % by weight):
The advantages of the invention can be seen, inter alia, in the fact that by optimisation of Al activity in the alloy and due to the-specific phase structure, consisting of fine precipitates of xcex3xe2x80x2 and xcex1-Cr in xcex3-matrix an improved ductility and creep resistance, phase stability of coating and substrate during service, phase structure and thermal expansion similar to the substrate and an excellent oxidation resistance can be obtained. To achieve the xcex3-xcex3xe2x80x2-xcex1-Cr-structure the relatively high but limited contents of Al and Cr were combined. To prevent coarsening of the xcex1-Cr phase an addition of more than 3% Re was necessary.